Machine washing systems typically employ cleaning agents that form a cleaning solution when mixed with water. Cleaning agents, such as detergents, breakdown and remove food or other soils and operate most effectively in alkaline environments. However, hard water ions (e.g., calcium and magnesium ions) present in the water supply of the washer will reduce the effectiveness of the detergent. Thus, chelating agents are added to the wash water to sequester the hard water ions. Typical chelants include, without limitation, nitrilotriacetic acid (NTA), ethylene diamene tetraacetic acid (EDTA) and tripolyphosphate (TPP).
The water hardness (i.e., the concentration of hard water ions) is typically not known prior to beginning a wash operation, so the amount of chelant that needs to be added to the wash water to sequester the hard water ions is also not known.
One approach to account for unknown water hardness is to add an excess amount of chelating agent to the detergent to sequester all possible hard water ions. However, this approach typically wastes a large amount of chelant. In addition, this approach is costly because chelating agents are expensive.
Two approaches to determining when enough chelant has been added to completely sequester the hard water ions are described in U.S. Pat. No. 4,334,881, entitled “Method and Apparatus for Automatic End-Point Detection in Tripolyphosphate Sequestration of Hardness.” In one approach, TPP is slowly added to the wash water while the electrical conductivity of the wash water is monitored. Complete sequestration is determined by detecting when the maximum rate of change of conductivity with chelant addition occurs. In the second approach, if the pH of the washing solution is above 11 (to precipitate the magnesium ions), when the conductivity first increases substantially with TPP addition, 1.5 times more TPP is added to sequester all of the remaining calcium ions. Both of these approaches, however, tend to be quite slow and inaccurate.
Thus, there is a need for improved methods and systems to measure water hardness and to determine the amount of chelating agent needed to sequester hard water ions.